The modify_date that you have found is the last time someone added a column, removed a column, changed a data type, etc. It is not the last time that someone changed data within the table.
SQL does not natively record the name of the last person to alter a table. You can setup a DDL trigger on the database to monitor this and log the information to a table.
Like wise SQL does not record the last person to modify data within the table either. You can setup a normal trigger on the tables of interest and log this information to a table.